1. Field of the Invention
The present invention relates to optical recording media, which being appropriately utilized for wavelength-selective reflection films in optical recording media of hologram type capable of high density image recording, methods of producing the optical recording media, as well as optical recording methods and optical reproducing methods that utilize the optical recording media.
2. Description of the Related Art
One type of recording media capable of recording large amounts of information e.g. high-density image data is optical recording media. These optical recording media have been commercially introduced, for example, in a form of rewritable optical recording media such as optical magnetic discs and phase change optical discs and in a form of recordable optical recording media such as CD-Rs. These optical recording media have been limitlessly demanded for their still larger capacity. However, all of optical recording media have conventionally been based on two-dimensional record, which forces a limit in a sense for increasing their recording capacity. Accordingly, optical recording media of hologram type are attracting attention in recent years that are capable of recording in three-dimensional fashion.
The optical recording media of hologram type typically record information in a way that an informing light having a two-dimensional intensity distribution and a reference light having approximately the same intensity with the reference light are duplicated at inside of a photosensitive recording layer, thereby causing a distribution of an optical property e.g. refractive index by use of an interference pattern formed by the lights. On the other hand, when recorded information is to be read or reproduced, only the reference light is irradiated onto the recording layer in the similar arrangement as at the recording, and the diffracted light having the intensity distribution corresponding to the optical property distribution formed inside the recording layer is emitted from the recording layer.
In the optical recording media of hologram type, the optical property distribution is three-dimensionally formed inside the recording layer, therefore, one region where information being recorded by an informing light and another region where information being recorded by another informing light can be partially overlapped, thus multiple recording can be realized. When digital volume holography synthesized by computers is available, the original information can be reproduced exactly even if the overwriting lowers somewhat the signal/noise ratio (S/N ratio) by virtue that the S/N ratio can be made remarkably higher at one spot. As a result, the multiple recording times may be extended into as long as several hundred times, and the recording capacity of optical recording media may be increased remarkably (see Japanese Patent Application Laid-Open UP-A) No. 2002-123949).
As for the optical recording media of hologram type, for example, optical recording medium 20 is proposed, as shown in FIG. 1, which is provided with servo pit pattern 3 at the surface of second substrate 1, reflective film 2 formed of aluminum etc. on the surface of the servo pit pattern, recording layer 4 on the reflective film, and first substrate 5 on the recording layer (see JP-A No. 11-311936).
However, there exists such a problem in the optical recording medium 20 having the construction shown in FIG. 1 that the recording density decreases by half due to the construction of the servo zone and recording zone being divided in a plane.
Accordingly, the optical recording medium 20a shown in FIG. 20 utilizes a circular light for the informing and reference lights, and is provided with gap layer 8 for smoothening the second substrate 1, cholesteric liquid-crystalline layer or dichroic mirror layer as filter layer 6, and one-4th wavelength plate 10; and the recording layer and the servo layer are superimposed in the thickness direction (see JP-A No. 2004-265472). This proposal may bring about doubling of the recording density. In addition, when the cholesteric liquid-crystalline layer of mono-layer is utilized as the filter layer that has the same rotating direction with the circular light of the informing light in the spiral configuration, the optical recording media can be mass-produced with higher productivity and lower cost, and also the filter effect may be proper at vertical incidence 0°.
In this proposal, however, noise may be induced by the change of incident angle, i.e. because incident light inclination of no less than 10° tends to cause a shift into the selective reflection wavelength, thus the informing light and the reference light reach the reflective film through the filter layer. This phenomenon leads to a problem that the optical recording media cannot be applied to conventional lens optical systems where the incident light is passes through a diaphragm and exhibits an inclination of no less than ±10°.
In the case of the incident angle being no more than 10°, the informing and the reference lights 35 are entirely subjected to selective reflection at filter layer 6 to yield a return light under normal conditions as shown in FIG. 3; on the contrary, the informing and the reference lights 35 may not be entirely reflected at the filter layer 6 as shown in FIG. 4, that is, a small amount of the lights may leak through the filter layer to reach the reflective film 2 and may reflect by the reflective film, then the resulting reflected light 35a may get mixed with the diffracted light thereby to cause noise.
Accordingly, the optical recording media of hologram type, capable of preventing noise even when the informing light and the reference light leak from selectively reflective layers, have not been mass-produced yet with higher efficiency and lower cost, thus it is currently desired that the production may be achieved as soon as possible.